Antibodies are highly useful molecules because of their ability to bind almost any substance with high specificity and affinity and their ability to remain in circulation in blood for prolonged periods as therapeutic or diagnostic agents. For treatment of humans, Abs derived from human Abs are much preferred to avoid immune response to the Ab. For example, murine Abs very often cause Human Anti Mouse Antibodies (HAMA) which at a minimum prevent the therapeutic effects of the murine Ab. For many medical applications, monoclonal Abs are preferred. Nowadays the preferred method of obtaining a human Ab having a particular binding specificity is to select the Ab from a library of human-derived Abs displayed on a genetic package, such as filamentous phage.
Libraries of phage-displayed Fabs and scFvs have been produced in several ways. One method is to capture the diversity of donors, either naive or immunized. Another way is to generate libraries having synthetic diversity. The present invention relates to methods of generating useful diversity in human Ab scaffolds. As is well known, typical Abs consist of two heavy chains (HC) and two light chains (LC). There are several types of HCs: gamma, mu, epsilon, delta, etc. Each type has an N-terminal V domain followed by three or more constant domains. The LCs comprise an N-terminal V domain followed by a constant domain. LCs come in two types: kappa and lambda.
Within each V domain (LC or HC) there are seven canonical regions, named FR1, CDR1, FR2, CDR2, FR3, CDR3, and FR4, where “FR” stands for “Framework Region” and “CDR” stands for “Complementarity Determining Region”. For LC and HC, the FR and CDR GLGs have been selected over time to be secretable, stable, non-antigenic and these properties should be preserved as much as possible. Actual Ab genes contain mutations in the FR regions and some of these mutations contribute to binding, but such useful FR mutations are rare and are not necessary to obtain high-affinity binding. Thus, the present invention will concentrate diversity in the CDR regions.
In LC, FR1 up to FR3 and part of CDR3 comes from a genomic collection of genes called “V-genes”. The remainder of CDR3 and FR4 comes from a genomic collection of genes called “J-genes”. The joining may involve a certain degree of mutation, allowing diversity in CDR3 that is not present in the genomic sequences. After the LC gene is formed, somatic mutations can give rise to mature, rearranged LC genes that have higher affinity for an antigen (Ag) than does any LC encoded by genomic sequences. A large fraction of somatic mutations occur in CDRs.
The HC V region is more complicated. A V gene is joined to a J gene with the possible inclusion of a D segment. About half of HC Abs sequences contain a recognizable D segment in CDR3. The joining is achieved with an amazing degree of molecular sloppiness. Roughly, the end of the V gene may have zero to several bases deleted or changed, the D segment may have zero to many bases removed or changed at either end, a number of random bases may be inserted between V and D or between D and J, and the 5′ end of J may be edited to remove or change several bases. Withal, it is amazing that human heavy chains work, but they do. The upshot is that the CDR3 is highly diverse both in encoded amino-acid sequences and in length. In designing synthetic libraries, there is the temptation to just throw in a high degree of synthetic diversity and let the phage sort it out. Nevertheless, D regions serve a function. They cause the Ab repertoire to be rich in sequences that a) allow Abs to fold correctly, and b) are conducive to binding to biological molecules, i.e. antigens.
One purpose of the present invention is to show how a manageable collection of diversified sequences can confer these advantages on synthetic Ab libraries. Another purpose of the present invention is to disclose analysis of known mature Ab sequences that lead to improved designs for diversity in the CDR1 and CDR2 of HC and the three CDRs of lambda and kappa chains.